The present invention relates to an electrophotographic printer, more particularly to an improved method of controlling an electrophotographic printer.
Widely used in copiers, facsimile machines, and computer systems, electrophotographic printers have a photosensitive drum that is illuminated to form a latent image. The latent image is developed by application of toner, which is then transferred to printing media, such as paper, passing between the photosensitive drum and a transfer roller. The toner adheres to the photosensitive drum because of electrostatic attraction, and is also transferred by electrostatic attraction to the printing media.
A major factor determining the quality of the printed image is the transfer current flux between the surface of the photosensitive drum and the interior of the transfer roller. If the transfer current is too weak, the transferred image will be faint or patchy. If the transfer current is too strong, electrostatic forces may scatter toner particles on the paper, creating a fuzzy image. The transfer current is affected by ambient conditions such as temperature and humidity, which alter the moisture content and hence the electrical resistance of the printing media and transfer roller, and must be regulated by, for example, adjusting the transfer voltage applied to the roller.
One conventional method of adjusting the transfer voltage measures the combined electrical resistance of the printing media and transfer roller at the instant when the front edge of a page is caught by the transfer roller, and adjusts the transfer voltage according to the measured resistance. A problem with this method is that the high-voltage power supply that generates the transfer voltage has a limited response speed, so in high-speed printing, the transfer voltage cannot be adjusted quickly enough to prevent degradation of the image at the top of the page.
Another conventional method equips the printer with a temperature-humidity sensor, and sets the transfer voltage to a value determined from the ambient temperature and humidity. One problem with this method is the high cost of the sensor. Another problem is that the inherent electrical resistance of the transfer roller varies from one manufactured lot of rollers to another, and also changes over the life of the printer, making it difficult to determine the correct transfer voltage from ambient conditions alone.
To complicate the problem, an electrophotographic printer has other components that are affected by ambient conditions and require adjustment of applied voltages.